Radio modulation system



Dec. 11 1923.

F. CONRAD RADIOMODULATION SYSTEM Filed July 11. 1922 W. m A m w r.

INVENTOR 'FvzmK Convad.

wlTNssssz ATTORN EY Patented Dec. 11, 1923*.

RADIO MODULATION SYSTEM.

Application filed July 11,

To all whom it may concern:

Be it known that I, FRANK Comm), a citi-. zen of the United States, and a resident of Pittsbur h, in the county of Allegheny and State of ennsylvania, have invented a new and useful Improvement in Radio Modulation Systems, of which the following is a specification.

y invention relates to radio telephone transmission and it has particular relation to means for measuring the modulation and to methods of and apparatus for controlling the modulation.

One object of my invention is to provide a meter registering the alternating-current component of the modulated current supplied ,to an oscillator tube, said meter being calibrated to indicate directly the ratio of the effective alternating-current component to .7 times the steady direct-current component.

A further object of my invention is to provide a mechanism and a method of op-. eration wherein the intensity of the modulation is controlled in accordance with a chart showing various average effective values of the modulating current or, in eneral, of the modulation envelope. uch values are worked out with respect to the different kinds of sound being transmitted in such manner that the loudest individual units of the sound shall not, in general, cause the instantaneous intens'ty of modulation to materially exceed the value at which distortion from over modula-tion begins, as more fully described hereinafter.

With these and other objects in View, my invention consists in the methods of operation and in the details of apparatus hereinafter described and claimed, and illustrated in the accompanying drawing, wherein Figure 1 is a diagrammatic view of circuits and apparatus embodying a preferred form of my invention, and

Figure 2 is a similar view showing a modification.

In order to make clear disclosure and to illustrate the principles of my invention, I shall describe it in detail with reference to a preferred form, although it is to be undersigned for a 1922. Serial No. 574,317.

stood that various modifications and substitutions ofequivalents known to those skilled in the art may be adopted without departting from the spirit and scope of the invenion.

In Fig. 1 is shown a conventional oscillation generator and modulation system comprising an oscillator tube 3 having a plate 4, a grid 5 and a filament 6, and an oscillator tube 7 havin a plate 8, a grid 9 and a filament 10. T e plate circuits of both tubes are connected-in parallel across a common source shown as a high-voltage battery 11, having a large choke-coil 12 in seriw therewit The grid circuit of the modulator tube is connected to suitable amplifier devices associated with a telephone circuit. The grid circuit of the oscillator tube is connected, through a grid condenser 13 and a grid leak 14, to variable taps 15 and 16 on a coil 17. The plate of the oscillator tube is also connected to said coil 17 through a stopping condenser 18. An antenna circuit, comprising an antenna 19 and a ground connection 20, is also connected to the coil 17.

The circuits so far described comprise a well known transmission system wherein the choke-coil 12 causes the source 11 to supply a substantially constant current and wherein modulation of the plate-current supplied to the oscillator tube 3 is effected by means of variable currents drawn by the plate circuit of the modulator tube 7. A radiofrequenoy choke-coil 21 is included in the connections between :the oscillator plate 4 and the source for preventing loss of radiofrequency energy.-

In order'to indicate the amount of modulation, a so-called modulatio meter has been developed. As shown, a current transformer is provided having an iron core 22 constructed of very thin laminations and deprimary current to secondary current over a wide range of frequency values. The transsubstantially constant ratio of former has a primary winding 23 connected in series with the supply circuit for the oscillator plate 4 may be connected to any instrument capable of correctly indicating the current throughout the desired frequency range.

A thermo-ammeter is shown comprising a heating element 25 terminals of the secondary winding 24, a thermo-couple 26 responsive .to the temperature of the heat ng element 25 and a galvanometer 27 connected across the terminals of the thermocouple. An air gap 22a is provided in the transformer core to prevent saturation by reason of the direct-current component of the plate current.

In the operation of a radio telephone transmission system, such as that described above, complete modulation occurs when the current drawn by the oscillator tube varies from zero to twice the steady current supplied by the source. However, faithfulness in reproduction usually requires that the modulation intensity shall not exceed, in general, about 40 or 50 percent of complete modulation. Heretofore, it has been practically impossible to obtain the desired condition at all times, both on account of the lack of convenient meters for indicating modulation intensities and on account of the fact that certain individual notes of the transmitted sound are louder than others, and this discrepancy varies widely according to the nature of the sound being transmitted.

I have found that, by measuring the average eifective value of the modulation current or of the modulation ratio, and by preparing a chart in advance, showing suitable average effective values for the different kinds of sound to be transmitted, a current indicator, such as that hereinabove described, may be utilized to secured the best modulation value in accordance with the kind of sound being transmitted and the corresponding values prescribed by said chart, as will be more fully hereinafter explained.

I have, therefore, calibrated my galvanometer 27 with a scale marked from zero to 100% modulatioil'r' In order to calibrate this meter, two methods are available. In the first method, the steady direct currentsupplied by the battery 11 is first measured. Then, the maximum permissible effective value of the alternating plate current would be .7 times the steady value of the direct current. The transformer windings 23 and 2A are then arranged to give such transformation ratio that a full-scale deflection of the galvanometer corresponds to the maximum alternating current.

The second method of calibrating the meter involves the use of an oscillograph. As sound is applied to the transmitting set, the oscillations in the detector circuits of the oscillograph are observed. When the os cillations reach 100% of the fixed component of the high-frequency oscillations, the

connected across the been determined by .average volume of meter should give full-scale deflection. It will be understood, of course, that the oscillograph, in this case, is either connected directly in the antenna circuit or coupled thereto. When the full-scale deflection has either of the above methods, the meter scale is then calibrated in percentage of modulation, taking 100% as the full-scale deflection, or any other system of calibration may be adopted as may be desired.

As hereinabove indicated, a chart is prepared indicating the various average values of the deflection necessary to produce the desired modulation results, in accordance with the different kinds of sound to be transmitted. Thus, in popular broadcasting service, when the announcer is speaking into the transmitter, the modulation amounts to about 40 percent, with maximum between and percent. Piano solos average about 30 percent, cent and vocal numbers 40 to 50 percent violin solos 20 to 30 per-,

with a maximum of 100 percent. Of course,

the modulation meter indicates only the sound. While the meter may read only 30 percent, in case of piano music, the individual notes at the instant of striking may reach 80 to per cent. In violin solos, the music is usually quiet, but loud chords are likely to occur which would result in bad distortion if the average percentage of modulation were not kept rather low. Allowing for the kind of sound being transmitted, that is, piano, speaking voice, solo, etc., the modulation meter provides a convenient means for finding the correct distance to place the artist from the pick-up transmitter, or for adjusting the amplification of the amplifiers.

In Fig. 2 is shown a slightly modified system in which the saturating effect of the direct-current component is avoided by employing a secondary winding 23' having a mid-tap 28 which is connected to the source of supply, one portion of the secondary winding being connected in the oscillator circuit and the' other portion in the modulator circuit. In this manner, the direct current normally flows in opposite directions to the respective plate circuits. The alternating components in the two plate circuits are opposite in phase and hence tbealternating components are accumulative and cause i means for causing said device to indicate the ratio of the actual modulation current to the maximum modulation current possible without undesirable distortion.

. 2. The combination with a conductor carrying a modulated direct current having a substantially constant unidirectional component and a variable alternating component, of a modulation meter comprising a current transformer associated with said conductor, a current-measuring device associated with said transformer and means for causing said device to indicate the ratio of the actual alternating-current component to the maximum value possible.

3. The combination with a conductor carrying a modulated direct current having a substantially constant unidirectional component and a variable alternating component, of a modulation meter comprising a current transformer associated with said conductor, a current-measuring device associated with said transformer and means for causing said device to indicate ratios of the effective values of the alternating-current component to a predetermined fraction of the constant unidirectional component.

4. The combination with a conductor carrying a modulated direct current having a substantially constant unidirectional component and a variable alternating component,

of a modulation meter comprising a current transformer associated with said conductor, a current-measuringdevice associated with said transformer and a scale for said device calibrated in ratios of the effective values'of the alternating-current component to .7 times the constant unidirectional component.

5. In a radio tern. the combination with a source of modulated direct current having a substantially constant unidirectional component and a variable alternating component. of an oscillation generator generating higl1-frequency currents modulated in accordance with said source. a current transformer associated with said source, a current measuring device associated-with said transformer and means for causing said device to indicate the ratio of the actual alternating-current component to the maximum value possible Without undesirable distortion.

In a radio telephone transmitting system, the combination'with a source of modulated direct current having a substantially constant unidirectional component and a variable alternating component, of an oscillation generator generating high-frequency currents modulated in accordance with said source, a current transformer associated with said source. a current-measuring device associated with said transformer and means for causing said device to indicate ratios ofthe effective values of the alternating-current component to constant unidirectional component.

telephone transmitting sys- .7 times the" former having an 7. In a radio telephone transmitting system, the combination with a source of modulated direct current having a substantially constant unidirectional component and a variable alternating'component, of an oscillation generator generating high-frequency currents modulated in accordance with said source, a current transformer associated with said source, a current-measuring device associated with said transformer and a scale for said device calibrated in ratios of theeffective values of the alternating-current component to .7 times the constant unidirectional component.

8. The combination with a conductor carrying a modulated direct current having a substantially constant unidirectional component and a variable alternating component, of a modulation meter comprising an iron-core current preventing saturation of said transformer core by reason-of said unidirectional component, a current-measuring device associated with said transformer and means for causing said device to indicate the ratio of the actual alternating-current component to the maximum value possible without undesirable distortion. 1 V

9. The combination with a conductor carrying a modulated direct current having a substantially constant unidirectional component and a variable alternating component, of a modulation meter comprising an ironcore current transformer associated with said conductor, the core of said transformer having an air-gap, a current-measuring device associated with said transformer and means for causing said device to indicate the ratio of the actual alternating-current component to the maximum value possible without undesirable distortion.

10. The combination with a conductor carrying a modulated direct current having a, substantially constant unidirectional component and a variable alternating component, of a modulation meter comprisin an iron-core current transformer assoclated with said conductor, means for preventing saturation of said transformer core by reason of said unidirectional component, a current-measuring device associated with said transformer and means for causing said device to indicate ratios of the effective values of the alternating-current component to .7 times the constant unidirectional component.

11. The combination with a conductor carrying a modulated direct current having a substantially constant unidirectional component and a variable alternating component, of a modulation meter comprising an iron-core current transformer associated with said conductor, the core of said transtransformer associated with said conductor, means for air-gap, a current-measuring device associated with said transformer and a scale for said device calibrated in ratios of the effective values of the alternating-current component to .7 times the constant unidirectional component.

12. In a radio telephone transmitting system, the combination with a source of modulated direct current having a substantially constant unidirectional component and a variable alternating component, of an oscillation generator generating high-frequency currents modulated in accordance with said source, an iron-core current transformer associated with said source, means for preventing saturation of said transformer core by reason of said unidirectional component, a current-measuring device associated with said transformer and means for causing said device to indicate ratios of the effective values of the alternating-current component to .7 times the constant unidirectional component.

13. In a radio telephone transmitting system, the combination with a source of moduconstant unidirectional component and a variable alternating component, of an oscillation generator generating high-frequency currents modulated in accordance with said source, an iron-core current transformer associated with said source, the core of said transformer having an air gap, a currentmeasuring device associated with said transformer, and means for causing said device to indicate ratios of the effective values of the alternating-current component to .7 times the constant unidirectional component.

14. In a radio telephone transmitting system, the combination with a source of modulated direct current having a, substantially constant unidirectional component and a variable alternating component, of an oscillation generator generating high-frequency currents modulated in accordance with said source, an responsive to the average effective value .of the alternating component of said modulated direct current, means for varying the intensity of modulation, and chart showing various values of the avera e alternating components desirable for different kinds of sound being transmitted, substantially as dei'scribed. g

15. The method of securing an effective utilization of an indicating instrument respons'ive to the average efiective value of the alternating component of a modulated direct-current supplied to an oscillation generator, in conjunction with a chart showing various values of the average alternating components desirable for different kinds of sound bein transmitted, substantially as described, sa1d method consisting in varying the intensity of modulation in such manner as to cause the indications of said instrulated direct current having a substantially,

indicating instrument ment to conform, in general, to the corresponding values prescribed by said chart.

16. In radio telephone transmission, the method of controlling the intensity of modulation of the direct current suppliedto an oscillation generator consisting 1n securing an indication responsive to the average effective value of the alternating component of the modulated direct current and varying the intensity of modulation in accordance with the information obtained from such indication.

17. In radio telephone transmission, the method of controlling the intensity of modulation of the direct current supplied to an oscillation genera-tor consisting in securing an indication responsive to the average effective value of the alternating component of the modulated direct current and varying the intensity of modulation in such manner as to cause said indications to conform substantially to predeterminedavera e values dependent upon the kind of soun being transmitted, said predetermined average values being so chosen for loudest individual notes of the sound shall not, in general, cause the instantaneous effective value of the alternating component to materially exceed .7 times the direct-current component.

18. In radio telephone transmission, the

method of controlling the intensity of modulation of the direct current supplied to an oscillation generator, consisting in causin the alternating component of the modulate l direct current to approximate certain predetermined avera e values, dependent upon the each kind of sound that the kind of sound eing transmitted, said predetermined average values being so chosen for each kind of sound that the loudest individual notes of the sound shall not, in general, cause the instantaneous maximum value of the alternating component to materially exceed the direct-current component.

19. A method of radio telephone transmission comprising the step of varying the intensity of modulation in such manner that the modulation envelope approximates cer-. tain predetermined average values relative to the steady unmodulated wave and dependent upon the kind of sound being trans mitted, said predetermined average values being so chosen for each kind of.sound that the loudest individual notes of the sound shall not, in general, cause the momentary intensity of modulation to materially exceed the value at which distortion from over-modulation begins, substantially as described.

20. A radio telephone transmission system comprising a radio-frequency source, means for modulating the same, an indicating instrument responsive to average values of the intensity of modulation, and a chart showing certain predetermined average values relative to the steady unmodulate wave and dependent upon the kind of sound being trans-' modulation begins, substantially as demitted, said predetermined average values scribed. being so chosen for each kind of sound that In testimony whereof, I have hereunto 10 the loudest individual notes of the sound subscribed my name this 5th day of July, 5 shall not, in general, cause the momentary 1922.

intensity of modulation to materially exceed the value at which distortion from over- FRANK CONRAD. 

